1. Field of the Invention
The present invention relates to a red fluorescent material; a white light emitting diode using the red fluorescent material; a lighting device, a light emitting diode display, and a backlight unit for a liquid crystal display, these devices and unit using the white light emitting diode; and a liquid crystal display using the backlight unit.
2. Description of the Related Art
Light emitting diodes are different in light emitting mechanism from currently used light fixtures such as incandescent bulbs and fluorescent bulbs, and are compact and bright; the electric power consumption of light emitting diodes is about one-eighth the electric power consumption of bulbs having the same brightness and about one-half the electric power consumption of fluorescent lamps having the same brightness, and thus, light emitting diodes have properties excellent in energy saving. Additionally, light emitting diodes are long in operation life, excellent in shock resistance, compact and lightweight, free from yielding such harmful wastes as derived from conventional light sources, and accordingly can be used as excellent light sources in harmony with the environment.
As white light emitting diodes, known are those disclosed in Japanese Patent Nos. 2900928, 2998696, 2927279, etc. which use: light emitting diodes using nitride semiconductors and emitting blue or blue-green light; and a fluorescent material in which Ce is doped in a YAG based oxide matrix lattice (YAG:Ce fluorescent material, hereinafter abbreviated as YAG based yellow fluorescent material) known by a composition formula of (Y,Gd)3(Al,Ga)5O12, the fluorescent material being excited by the blue or blue-green light emission of the above described light emitting diodes to emit yellow fluorescence to be complementary to the light emission of the light emitting diodes. In white light emitting diodes of this type, fluorescent materials are dispersed in the sealing resin portion enclosing each light emitting diode. Additionally, Japanese Patent Laid-Open No. 11-46015 discloses a white light emitting diode in which a non-particulate fluorescent material layer is formed as a film on a blue light emitting diode.
Additionally, National Publication of International Patent Application No. 2000-509912 discloses a three-wavelength type white light emitting diode in which are combined three ultraviolet light emitting fluorescent materials respectively emitting red light (590 nm to 630 nm), green light (520 nm to 570 nm) and blue light (430 nm to 490 nm) caused by an ultraviolet light emitting diode emitting short wavelength near ultraviolet light (370 to 410 nm). The white light emitting diode includes an ultraviolet light emitting diode disposed inside a transparent resin portion formed in a dome shape on a transparent substrate (a front panel). In the transparent resin portion are admixed three types of fluorescent material powders respectively emitting red, green and blue light caused by ultraviolet light. The surface of the transparent resin portion is subjected to mirror finish so as to work as a mirror.
National Publication of International Patent Application No. 2000-509912 discloses Y2O2S:Eu3+ as a red light emitting fluorescent material with an ultraviolet emitting diode composed of InGaN or GaN as exciting light source.
These white light emitting diodes are low in electric power consumption and long in operation life, and hence have began to be used in the fields of lighting systems, displays, liquid crystal display and the like.
White light emitting diodes, using the blue or blue-green light emission and yellow fluorescence complementary to the blue or blue-green light emission, are used for backlight units of liquid crystal display and for light emitting diode displays. However, the white light emitting diodes having such structures as described above are poor in color reproducibility and color rendering properties because of insufficient light emitting intensity in the red region; thus, improvement of these problems have been demanded.
Additionally, a white light emitting diode using an ultraviolet light emitting diode, disclosed in National Publication of International Patent Application No. 2000-509912, has a high light emission efficiency in the light emission wavelengths from 370 nm to 410 nm of the ultraviolet light emitting diode, and particularly, has a highest light emission efficiency at wavelengths around 390 nm. On the contrary, a red light emitting fluorescent material of Y2O2S:Eu+3 efficiently absorbs light of 370 nm wavelength, and hence the light emission efficiency is not sufficient when an ultraviolet light emitting diode is used as an excitation source; thus, demanded is a material which can be an ultraviolet light emitting diode for the wavelength around 390 nm and has a further higher light emitting efficiency.